1. Field of the Invention
The present invention relates to neutron imaging, and more specifically, it relates to phase-sensitive neutron imaging and absorption-based neutron imaging.
2. Description of Related Art
Neutron imaging, and, more specifically, phase-sensitive neutron imaging and absorption-based neutron imaging, can provide detailed material as well as spatial information of samples on an atomic level for a variety of applications, ranging from biological imaging to non-destructive testing. This diagnostic complements other radiographic modalities, examples of which include x-ray, THz, e-beam, optical coherence tomography and MRI imaging systems. These systems differ fundamentally owing to the interaction of the specific probe beam with the electronic and/or nuclear properties of a given material. Furthermore, the wavelength of an EM wave or the de Broglie wavelength of a particle provide constraints in terms of the scale size of various features or defects that can be imaged, and moreover, the depth in a given workpiece to which the diagnostic can reveal meaningful information.
The prior art in phase-sensitive neutron imaging techniques includes single crystal interferometry, diffraction-enhanced imaging, phase-contrast imaging and Moiré deflectometry, to achieve higher sensitivity over absorption only measurements. The interferometric class of diagnostics requires high temporal coherence of the incident neutron probe; and, diffraction-enhanced imaging systems require many angular rotations to fully map out the phase distribution. Phase-contrast imaging requires multiple images to detect all of the spatial scales necessary to obtain the desired information. Finally, at present, Moiré deflectometers which have been implemented to date, only measure the gradients in one dimension, and therefore, require multiple measurements in orthogonal directions to obtain the entire two-dimensional field.
In the case of wavefront sensing in the visible (optical) regime, prior art exists, including a two-dimensional shearing interferometer based on crossed phase gratings. As an example, a crossed phase grating in the optical domain was formed by etching a “chessboard” (or, equivalently, a “checkerboard”) pattern of alternating optical phase shifts into a glass substrate. Moreover, in the visible regime, prior art exists based upon two-dimensional Hartmann sensors using, as an example, phase screens and lenslet arrays to extract the wavefront of an incident optical beam to the system. However, the implementation of these wavefront-sensing techniques to the domain of neutron wavefront sensing, has not, to Applicant's knowledge, been considered.
A shearing interferometer is a diagnostic tool that enables one to determine the shape of a wavefront, or, equivalently, its spatial phase map, by producing an intensity pattern consistent with the gradient of the equiphase surfaces of the incident beam. The intensity pattern results from the interference of the incident beam, with an angularly displaced replica of itself. Hence, the gradient of the phasefront is effectively transformed into an intensity map. In the case of a plane wave, the resultant intensity pattern consists of a set of parallel fringes. In the case of a converging or diverging beam, the intensity pattern consists of concentric rings, consistent with the curvature of the equiphase surfaces of the incident beam, etc. In the case of making the source spatially coherent, prior art include the use of an aperture or pinhole placed between the neutron source and the object as well as a 1-D periodic amplitude mask.